1. Field of the Invention
The present invention relates to injection molding machines, and, more particularly, to a hydraulic actuator assembly which serves to open and close the two halves of an injection molding die in the die closing unit of the machine.
2. Description of the Prior Art
The hydraulic actuator assembly of a die closing unit has to fulfill two major functions, one being to open and close the injection molding die as rapidly as possible over an opening stroke which is adjustable in accordance with the axial depth of the die cavity, the other being to exert a high clamping force on the closed injection molding die. These two functions require different hydraulic characteristics: The high clamping pressure requires a large piston diameter, whereas the opening and closing travel calls for a relatively small pressure surface, in order to attain rapid movements without the need for inordinate fluid pumping capacity.
To obtain this dual function in a hydraulic actuator with a maximum of efficiency, it has already been suggested to use a controllable flow bypass arrangement in connection with the power piston of the main hydraulic cylinder, so that the hydraulic fluid, instead of being displaced by the power piston during the opening and closing travel, flows through or around the moving power piston. This piston then becomes either a differential piston with a small effective pressure surface, or it is completely neutralized, while a small auxiliary piston provides the force for the opening and closing movements.
In my U.S. Pat. No. 3,935,791, I have disclosed several embodiments of a hydraulic actuator assembly which features a bypassable power piston and an auxiliary travel piston. The two pistons are arranged on a common piston rod. The bypass of the power piston is accomplished by means of a series of bypass channels which extend through the body of the power piston, from one axial side to the other, and are enclosed on one side within a concentric valve seat, so that the axial movement of an annular valve plunger on the piston rod opens and closes the bypass channels.
This type of power piston with valve-controlled bypass can be used in connection with both pull-type and push-type die closing units. (In a pull-type die closing unit, the stationary die half is arranged axially between the movable die half and two or more pulling hydraulic actuators whose piston rods also serve as tie rods. In a push-type die closing unit, the movable die half is arranged axially between the stationary die half and a coaxially aligned hydraulic actuator whose piston rod pushes the injection molding die closed.) In both cases, the annular valve plunger is arranged in the high pressure space of the power cylinder, i.e. on that side of the power piston which has to be pressurized, in order to obtain the elevated closing pressure.
The aforementioned patent discloses several configurations for the power piston and the cooperating bypass valve plunger, including a hydraulic supply line which leads axially through the piston rod to a plunger pressure space, for the hydraulic initiation of an axial movement of the annular valve plunger. The plunger pressure space is in all cases so arranged in relation to the configuration of the valve plunger that its pressurization opposes the effect of hydraulic pressure in the high pressure space of the power cylinder. The interaction between these two pressure spaces controls the opening and closing movements of the annular valve plunger. The pressure inside the high pressure space of the power cylinder produces a valve-opening action, when the annular valve plunger is designed as a piston with an outer sliding diameter larger than the diameter of the valve seat. A spring may be used to influence the axial movement response of the valve plunger. In this case, the plunger pressure space is arranged to create a valve closing movement of the valve plunger under pressurization. Alternatively, the shape of the annular valve plunger may be such that the pressure inside the high pressure space creates a valve closing movement of the valve plunger, in which case the plunger pressure space is arranged to create an opposing valve opening movement.
The use of an axially movable annular valve plunger in the high pressure space of the power cylinder makes it possible to attach the power piston to the piston rod, and it also lends itself for an arrangement of the bypass channels in the body of the power piston in such a way that a smooth flow configuration is obtained. A threaded sleeve on the piston rod provides a clamping action for the power piston, while also serving as a guide sleeve for the annular valve plunger. A larger diameter of the piston rod in the low pressure space of the power cylinder not only provides a clamping shoulder for the power piston, but also serves to convert the hydraulic actuator into a differential-piston actuator, when the bypass channels are open.
From my U.S. Pat. No. 3,663,140, it is also known to arrange two hydraulic supply lines in the piston rod of a bypassable power piston, for the generation of bypass opening and closing movements. In this case, however, these movements are executed by the power piston itself which is arranged to axially move in relation to the piston rod, thereby producing the desired valve action in cooperation with a collar formation on the piston rod. The possibility of being able to control the opening and closing movements of the bypass valve, independently of the pressure conditions in the pressure spaces of the power cylinder, brings with it a greater freedom of hydraulic programming for the actuator assembly. It may be desirable, for example, to generate a short burst of high power at the beginning of the die opening stroke, in cases where the configuration of the mold cavity is such that the finished part creates a high frictional resistance during the initial separation of the die halves.